Boring tools are known, comprising drill bits to which interchangeable turnover plates are fastened which are formed with the main cutting edges. These turnover plates are fastened to the drill bit by screws extending with their axes approximately in a radial plane of the drill bit and passing approximately through the center of the turnover plates.
This manner of fastening gives the turnover plate a rather great length in the direction of advance of the drill, and this may lead to disturbance in the discharge of the chips. With these known turnover plate drills, it belongs to the state of the art to have an asymmetrical arrangement of one, two, or more turnover plates, which provides for dividing the cut of the total cross section to be cut. This division of the cut makes it impossible for the drill to self-center and guide itself. As a consequence, such drills usually can be used only for drilling depths up to three times (in exceptional cases up to five times) the diameter of the boring tool. It is always only one cutting edge which produces the drilling diameter.
The great structural length in feeding direction at 0.degree. cutting angle sets rather insufficient narrow limits for the possibilities of chip formation by corresponding selectable geometries. Therefore, ideal conditions as to the cutting edge geometry for any material to be machined exist in rare cases only.